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Unformatted text preview: Chromosomes and Inheritance - 1 Chromosome Theory of Inheritance Although Gregor Mendel did tremendous work in determining how genetic information was passed from generation to generation, he had no knowledge of chromosomes or the process of meiosis or that genes, the units of heredity, were located on chromosomes. Particularly important, we know today that we have far more genes than chromosomes. Mendel only addressed genes (his "paired factors") that were independently inherited, and fortuitously, each of the traits he studied was on separate chromosomes in the pea plant. Walter Sutton and Theodor Boveri made the correlation between Mendel's conclusions about genes (the inheritable traits of Mendel's "paired factors") and the behavior of chromosomes during mitosis and meiosis in 1902. Sutton is credited with first proposing the chromosome theory of inheritance, in which he connected Mendel's "paired factors" with the behavior of homologous chromosomes during meiosis. Chromosomes are in pairs and genes, or their alleles, are located on chromosomes. Homologous Chromosomes separate during meiosis so that alleles are segregated. Meiotic products have one of each homologous chromosome but not both. Fertilization restores the homologous pairs of chromosomes. Chromosomes and Segregation of Alleles in Meiosis Chromosomes and Inheritance - 2 Genes Inherited Together Gene Linkage on Chromosomes From the time of Sutton's deduction that inheritance followed the behavior of chromosomes, genetic research had a "problem" to solve. Mendelian inheritance "required" genes to be inherited independently of each other, but there are far more genes than chromosome pairs. How can we have thousands of genes and not thousands of chromosomes? In 1908, researchers discovered a dihybrid cross in sweet peas that did not have the predicted Mendelian ratio of 9:3:3:1. 75% of the second- generation offspring had the same two traits as the dominant parent and 25% the same two traits found in the recessive parent, the 3:1 ratio expected for a monohybrid cross. But if flower color and pollen length (the genes observed) were on the same chromosome they could explain the 3:1 inheritance ratio. Since Sutton had hypothesized that we inherit chromosomes, it was logical to conclude that all of the genes on a chromosome are physically inherited together as a single linked group. Only genes that are located on different chromosomes have independent assortment during meiosis I and follow Mendel's independent assortment ratio (9:3:3:1 or 1:1:1:1 for a test cross). The inheritance ratio for linked genes is the same as the Mendelian 3:1 ratio for a monohybrid cross, or 1:1 for a test cross. For purposes of meiosis and inheritance all the genes on a chromosome are one connected (or linkage) unit....
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This document was uploaded on 01/06/2012.
- Winter '09